Acidity, metal hydride complexes

However, since the goal of this work was the synthesis of alcohols from olefins via hydrohydroxymethylation (75, 76), little attention was given to developing a shift-catalyst per se. Pettit has recently reexamined some of this work and shown that, by careful control of the pH of the reaction mixture, systems based on either Fe(CO)5 or Cr(CO)6 can be developed for the production of either formic acid or methanol from carbon monoxide and water (77, 78). Each of these latter systems involves the formation of metal hydride complexes consequently, molecular hydrogen is also produced according to the shift reaction [Eq. (16)]. 

The concept has been extended to the C-H activation of nitriles, which coordinate strongly with metals. As shown in Scheme 3, coordination of a nitrile to low-valent metal complex (M) would increase both the basicity of the metal complex and the acidity of the C-H bond adjacent to the nitrile, and hence oxidative addition of the metal into the a-C-H bond of the nitrile would occur readily to afford an a-cya-noalkyl metal hydride complex (7), which undergoes isomerization to a N -bonded nitrile complex (8). The reaction of 8 with an C-electrophile forms a carbon-carbon... 

Kinetic studies made on [Pd(PP2)(PEt )[(BF )2> and reported elsewhere (45), indicate that the rate of catalysis is first order in CO2, first order in catalyst, and first order in acid at low acid concentrations. These results are consistent with the mechanism shown in Scheme 2. In comparison with Scheme 1, two important features should be noted. First in Scheme 2, the formation of a coordinatively unsaturated metal hydride complex is necessary for CO2 insertion to occur. A priori there is no way of knowing whether or not the generation of a coordinatively unsaturated metal hydride will be required for catalysis since evidence exists for both associative and dissociative pathways for CO2 insertion into metal hydride and metal carbon bonds (20-25). This is the reason that complexes of the types... 

There are only limited pKj, scales for transition metal hydride complexes compared to the extensive scales for organic and inorganic acids, despite the fact that hydrides often mediate organometallic reactions these were reviewed in 1991 [42]. Most of these complexes contain carbonyl and/or phosphine ligands and are usually insoluble in water. The for about 20 neutral hydrido-carbonyl... 

We and others have found that CH2CI2 is an excellent solvent for acidic dihydrogen and dihydride compounds. Methylene chloride has a low dielectric constant so that monocationic complexes will usually form 1 1 ion pairs below 0.01 M and higher aggregates above this concentration. Several pK/ of metal hydride complexes in CH2CI2 anchored to the pKa fl of phosphonium salts or other acids have been reported (e.g. see Table 1.2 from our work) they fall in the range from about -5 to 12 [23,30,49-52]. These are actually ion-pair pK values because they have not been corrected for the effects of ion-pairing. 

A difference from previous studies of metal hydride complexes is that we have created a continuous ladder of overlapping acid/base equilibria. This involved finding a series of phosphoms-containing compounds with pK that differ by less than 2 units, the limit of accurate determination of equilibrium constants K. We have determined the equilibrium constants, K, for cationic acid- neutral base reactions (eq 13) or neutral acid-anionic base reactions (eq 14) in THF or THF-dg by use of quantitative 3 P gated H and H NMR. 

The use of stoichiometric ionic hydrogenations in organic synthetic applications was estabUshed prior to the use of transition metal hydride complexes for ketone hydrogenations [36, 37]. In traditional stoichiometric ionic hydrogenations, CF3CO2H is used as the acid and HSiEts serves as the hydride donor, though other acids and other hydride sources have been used as well. The C=0 bonds of ketones can be hydrogenated by this method, as well as certain types of C=C bonds. 

Kristjansddttir, S. S. Norton, J. R. Acidity of Hydrido Transition Metal Complexes in Solution. In Transition Metal Hydrides Recent Advances in Theory and Experiment, A. Dedieu, Editor VCH New York, 1990. Tables with pAa values of some transition metal hydride complexes. 

One-electron oxidation of transition metal hydride complexes significantly increases their acidity—their pX declines by about 20 units. For example, the pX of CpW(CO),(PMe3)H is 26.6 in CH3CN, whereas that of [CpW(CO)3(PMe3)H] is only 5.1 in CHjCN. ... 

In view of the acidity of hydride complexes, one might expect them to form hydrogen-bonded intennediates during proton-transfer reactions. However, it is relatively uncommon for neutral hydride complexes to form hydrogen bonds M-H A because most M-H bonds are polarized as M(8+)-H(8-), not as M(8-)-H(8+). (The need to reverse the observed polarization during deprotonation is a major cause of the low kinetic acidity of transition metal hydrides, mentioned previously.) The first M-H Ahydrogen bond from a neutral hydride has just been reported CpM(CO)3H (M = Mo and W) serves as a hydrogen bond donor to (octyl)3P=0 and even to pyridine, apparently because there is M(8-)-H(8+) polarization in its M-H bond. °... 

Hydroxide ion attacks metal-carbonyl complexes to form hydroxycarbonyl complex-es. These complexes are generally unstable and extrude COj to produce metal-hydride complexes (Equation 11.4). When the anionic hydride is sufficiently acidic to be deproto-nated by the hydroxide, an overall two-electron reduction of the metal results. Thus, CO... 

This transformation is catalyzed by metal-hydride complexes such as RuHCl(CO)(PPh3)3 as shown in the cycloisomerization of 17 into 18 (Eq. 3) [12], but can also be promoted by in situ generated metal-hydride species resulting from the interaction of non-hydridic complexes with acidic coreagent. This is illustrated by the cycloisomerization of aUyl propargyl ethers 19 into dienes 20 in the presence of catalytic amounts of RuCp Cl(COD) in acetic acid (Eq. 4) [13]. 

Copper-metal bonds have been synthesized by the condensation of (NHC)CuOFt with the acidic metal hydride CpMo(H)(CO)3 [171], or by halide displacement from (NHC)copper(l) using anions such as CpFe(CO)2 and CpMo(CO)3" (Cp = ri -CjHj Scheme 11.6) [172]. The net metal-metal interaction in these complexes is a a-bond In (lDipp)Cu-Fe(CO)2Cp, with a Cu-Fe distance of only 2.3462(5) A, the HOMO—1 and HOMO are Fe- Cu ic orbitals, and the LUMO is the a orbital. Natural population analysis indicates significant ionic character, consistent with unequal sharing of an... 

The charge plays a key role for the acidity and hydride-transfer property of metal-hydride complexes. Tilset has shown that the monoelectronic oxidation of a metal hydride lowers its pKa by 20.6 units for a variety of structures ... 

It is as difficult to distinguish between direct H transfer and elctron transfer followed by H atom transfer (H = e -i- H ) as it is to distinguish between direct H atom transfer and electron transfer followed by proton transfer (H = e" -t H+). It is possible to favor the electron-transfer reaction by using an anode or a powerful monoelectronic oxidant that is not an H atom acceptor. We have noted above that the acidity of a metal hydride is enormously increased by monoelectronic oxidation. Indeed, the monoelectronic oxidation of a neutral 18-electron metal-hydride complex to a 17-electron radical cation is systematically followed by the... 

In the same way as protonation of a neutral metal-hydride complex yields a cationic H2 complex, protonation of methane yielding transient CHs+ may be considered as protonation of a C-H bond leading to a transient H2 complex of the carbocation CH3+, a strong Lewis acid. 

Reductive aldol reaction of a,(5-unsaturated esters and enones with aldehyde mediated by a transition metal hydride complex and a hydride source, such as hydrosilane, is a versatile process to produce p-hydroxy carbonyl compounds (Scheme 15a) [21]. This reaction is thought to be an alternative transformation of Lewis acid-catalyzed Mukaiyama-type aldol reaction with silyl enol ethers or silyl ketene acetals (Scheme 15b). 

Hydrogenation of CO2 to formic acid (H2 -I- CO2 -< HC(O)OH) catalyzed by transition metal complexes is one of interesting and attractive reactions for COj fixation. Experimentally, various metal complexes, especially those of Ru and Rh, have been found to be able to efficiently catalyze this reaction [10]. Studies show that metal hydride complexes display excellent catalytic activity. This is understandable as reactions of metals with Hj easily generate metal hydride species. Computationally, hydrogenation of COj to formic acid is also among one of the most studied reactions involving COj [11,12]. 

A traditional method for such reductions involves the use of a reducing metal such as zinc or tin in acidic solution. Examples are the procedures for preparing l,2,3,4-tetrahydrocarbazole[l] or ethyl 2,3-dihydroindole-2-carbox-ylate[2] (Entry 3, Table 15.1), Reduction can also be carried out with acid-stable hydride donors such as acetoxyborane[4] or NaBHjCN in TFA[5] or HOAc[6]. Borane is an effective reductant of the indole ring when it can complex with a dialkylamino substituent in such a way that it can be delivered intramolecularly[7]. Both NaBH -HOAc and NaBHjCN-HOAc can lead to N-ethylation as well as reduction[8]. This reaction can be prevented by the use of NaBHjCN with temperature control. At 20"C only reduction occurs, but if the temperature is raised to 50°C N-ethylation occurs[9]. Silanes cun also be used as hydride donors under acidic conditions[10]. Even indoles with EW substituents, such as ethyl indole-2-carboxylate, can be reduced[ll,l2]. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

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